1. Field of the Invention
The present invention generally relates to an apparatus for reproducing an optical record medium. More particularly, it relates to an apparatus for reproducing an optical record medium to which information, having a spatial frequency higher than a spacial frequency prescribed by a wave length of a reading light and a numerical aperture of an objective lens at the time of reproduction, is recorded.
2. Description of the Related Art
Information corresponding to the existences of the phase pits of an optical disk, such as a CD (Compact Disk) and a LD (Laser Disk), is reproduced by detecting the reduction in the reflection light amount, which is caused by a light diffraction, a light scattering and a change in the optical constant of pit portions when the light spot of the reading light is formed on the pit portions, by use of an optical detecting device. More concretely, the information is reproduced based on the following facts. Namely, on one hand, when the reading light spot is positioned on the pit, the amount of the returning light is small because of the diffraction etc. On the other hand, the amount of the returning light is large when the reading light spot is positioned on the non-pit portion between the pits.
This kind of optical disk has the following problems. Namely, the reproduction resolution of the optical disk is restricted by a wavelength .lambda. of a reading light such as a reading laser beam, and a numerical aperture NA of an objective lens at the time of reproduction. The information, which includes a frequency component exceeding the spatial frequency fc=2 NA/.lambda., cannot be reproduced from the optical disk.
To overcome the above problems, an apparatus for reproducing an optical record medium, to which information having a spatial frequency higher than a spacial frequency prescribed by a wave length of a reading light and a numerical aperture of an objective lens at the time of reproduction is recorded, was developed. In order to make a reproduction of the information having a frequency component exceeding the spatial frequency fc=2 NA/.lambda., hereinafter referred to as "super resolution reproduction", a low temperature domain is produced within a light spot of a light beam emitted from a laser diode on the optical record medium.
However, the size of the low temperature domain is varied because of the differences of the light sensitivity of the material used for the optical record medium, room temperature, the power of the light beam etc. Therefore, since the size of the low temperature domain is varied because of the above mentioned reasons, the voltage of the reproduction signal is unstable, and the amount of crosstalk in the reproduction signal and error rate included in the reproduction signal are increased. As a result, the super high resolution reproduction becomes unstable.